1. Field of the Invention
This invention relates to a power steering apparatus utilizing a torsion bar.
2. Description of the Prior Art
In conventional power steering apparatus utilizing a torsion bar, it is so constructed that a feeling from a steering wheel is determined by a torsion resistance of this torsion bar. However, when the steering wheel is in its neutral position, there occurs little torsion resistance in the torsion bar, therefore its neutral rigidity becomes weak.
Thus, various types of apparatuses for enhancing the neutral rigidity have been proposed. For example, apparatuses disclosed in Japanese Patent Application Laid-Open No.Hei6-171520 and Hei3-139470 have been well known since before.
In the apparatus disclosed in Japanese Patent Application Laid-Open No.Hei6-171520, the input shaft is inserted into the output shaft so that a space is formed between both the shafts, and a ring-shaped spring is incorporated in this space. This spring is formed by cutting one position of the material and can be opened to the right and left from that cut position. Then, pins are protruded to both sides, the input shaft side and the output shaft side, and the respective pins are inserted into the cut positions of the above spring members.
If the input shaft and the output shaft are rotated relative to each other with the above condition, the ring-shaped spring members are opened from the cut positions. Reaction of the spring which occurs when they are opened acts as a preset force to enhance the neutral rigidity.
In the apparatus disclosed in Japanese Patent Application Laid-Open No.Hei3-139470, the input shaft is inserted into the output shaft and protrusions are formed on portions opposing each other of both the shafts. A spring force of the leaf spring is applied to these protrusions in such a condition that they are nipped.
If both the shafts are rotated relative to each other, the protrusions go contradictorily. When the protrusion are contradictory to each other, a spring force of the leaf spring acts. A preset force is exerted by this spring force.
The invention disclosed in the above Japanese Patent No.Hei6-171520 has such a problem that the spring force of the ring-shaped spring member cannot be specified easily.
Further, if its preset force is intended to increase, a deflection amount initially set must be increased. However, if the deflection amount initially set is increased, the spring members must be assembled at a predetermined position in such a condition that they are largely deflected. Assembly of the spring members in the deflection state worsens the assembly performance thereof.
On the contrary, if the amount of deflection initially set is decreased, the spring constant must be increased correspondingly. If the spring constant is increased, spring stress is increased correspondingly so that the spring may be broken in normal application range.
In the invention disclosed in Japanese Patent Application Laid-Open No.Hei3-139470, if the protrusions on both the shafts are not set symmetrically, preset force is deviated or looseness occurs. However, if it is intended to arrange them symmetrically, quite high an accuracy is needed, and therefore productivity of this product is reduced.